Yes a virtual tape volume can be accessed only by one client at a time
and if two processes/clients try to access the same volume at the same
time one process/client must wait.  Again smaller volume sizes decreases
the chance that a contention would happen and also decrease the
contention duration.

Why a VTL?  With us we found that when we out grew our physical library
we would have to have to buy over 30 physical drives in order to be able
to do backups, restores, cut off-site tapes and reclaim on/off site
tapes in the time allowed.  That amounted to some serious money, more
than our VTL costs.  When you also take into account the costs of the
much larger DISKPOOL a physical tape library requires, growing a
physical tape library in a TSM environment is not cheap.  The VTL
footprint is also smaller which also should considered in the total cost
of ownership.  Sorry, but we had to justify our VTL purchase.

Thanks,
H. Milton Johnson
-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Nicholas Cassimatis
Sent: Tuesday, June 12, 2007 10:55 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: [ADSM-L] Fw: How to Incorporate a CDL into TSM environment?

I'm not looking at the spinning through the volume to find the file, I'm
focused on the fact that a volume can only be accessed by one client at
a time.  You have to read the data to be restored, which takes time.  If
you have one client reading the volume, any other access to that volume
has to queue up.  With a slow client (or a fast one pulling a large
file), you can develop some access contention, which is a bottleneck
that collocation resolves.  That's why I still see collocation playing
with VTL's.

It all comes back to "Why do you want a VTL?" which is another way of
asking, "What problem are you trying to solve/avoid?"  I'm sure there
are people who are getting VTL's because they have to spend their budget
or they lose it - and the rest of us are jealous of them for that!  But,
as with most other technologies, implementing a VTL just moves the
bottleneck/weakest link to another spot, which may not be the best
solution for a given environment.

Nick Cassimatis

----- Forwarded by Nicholas Cassimatis/Raleigh/IBM on 06/12/2007 11:41
AM
-----

"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU> wrote on 06/12/2007
11:13:30 AM:

> As I see it, there are two areas where you get performance hits when 
> restoring from non-collocated volumes:
> 1) Tapes Mounts:  In my experience my VTL makes this problem 
> insignificant.
>
> 2) Spinning Sequential Media:  Yes, VTL volumes are sequential and if 
> you define your tapes as 50GB native and then with compression get 
> 100GB written to the tape, you may have to spin through 99.9GB of data

> to retrieve a 0.1Gb file.  However if you define 10GB volumes you only

> have to spin through 1/5 of the data to reach your 0.1GB file.  Also 
> with smaller volumes you are more likely to get "natural collocation" 
> because a client that writes directly to tape is more likely to fill
up a tape.
> Obviously if you define smaller and smaller volumes at some point you 
> will have a "tape mount bottle neck".
>
> Just one way to manage the trade offs.
>
> H. Milton Johnson